This invention pertains to machines for dressing grinding wheels. In particular, the invention concerns a method and apparatus for accurately locating, or tramming-in, a dressing wheel in a dressing machine used for form-dressing grinding wheels. Certain grinding operations require a grinding wheel having a particular surface contour, such as grinding of form cutters. In dressing such grinding wheels it is necessary to employ a dressing machine which moves a dressing wheel in a predetermined path about the periphery of the grinding wheel so that the dressed grinding wheel has the particular contour desired. This is often referred to as form-dressing and dressing machines having such capabilities are well-known.
A typical dressing wheel has sides which taper to a rounded circumferential apex and is provided with an abrasive material, such as diamonds, on the circumferential surface which contacts the grinding wheel being dressed. In a section is taken along the diameter of the dressing wheel at the point of dressing contact (the point of contact between the dressing wheel and the grinding wheel surface being dressed) the rounded circumferential apex is preferably defined by a given transverse radius having its center lying on a diametral axis of the dressing wheel.
During the dressing operation, the dressing wheel is preferably moved along the contour of the grinding wheel in a manner which maintains the center of the transverse radius a substantially constant distance from the surface of the grinding wheel. Additionally, the dressing wheel may be rotated at the center of the transverse radius about an axis perpendicular to, and lying in a common plane with, such diametral axis. The degree of rotation is normally limited so that any point on the dressing wheel coming in contact with the surface of the grinding wheel during dressing lies on the radial surface of the apex. This insures that all points of the dressing wheel which contact the surface being dressed are equi-distant from the center of the transverse radius so that accurate dressing can be accomplished.
In the typical dressing machine, a dressing wheel fixture is provided which has an axis, hereafter referred to as the dressing axis, which is moved along the contour of the grinding wheel being dressed at a fixed distance from the surface of the grinding wheel. The dressing wheel fixture is also adapted to rotate about the dressing axis. The dressing wheel is positioned in the dressing wheel fixture in fixed relationship to the dressing axis such that its circumferential apex contacts the grinding wheel during the dressing operation. As the dressing axis is moved about the contour of the grinding wheel contact between the rotating dressing wheel and the grinding wheel dresses the grinding wheel surface. The dressing wheel fixture is normally adapted to rotate about the dressing axis in a manner which similarly rotates the dressing wheel about the dressing axis so as to avoid interference between the dressing wheel and the grinding wheel surface being dressed.
If the center of the transverse radius is offset from the dressing axis the grinding wheel will not be accurately dressed since the distance between the circumferential surface of the dressing wheel and the surface of the grinding wheel will not be maintained constant. The degree to which this offset affects dressing accuracy depends on the tolerances of a particular dressing operation. The required size of the transverse radius generally depends on the characteristics of the contour being dressed. In certain grinding operations inside angles on the contoured surface may be such that the transverse radius of the apex must be very small, for example 0.005 in., in order to prevent undesirable gouging of the surface being dressed. In such instances it may be necessary to tram-in the dressing wheel so that the center of the transverse radius is located on the dressing axis within close tolerances, such as .+-.0.0001 in. Further, in any situation where an accurately dressed form is required it is necessary to accurately tram the dressing wheel since the accuracy of the dressed form is directly related to the accuracy of this center location.
From the foregoing it is apparent that it is necessary to tram-in or precisely locate the dressing wheel in the dressing wheel fixture of the dressing machine so that all points on the circumferential surface of the dressing wheel which may come in contact with the grinding wheel being dressed are equi-distant from the dressing axis. This requires that the center of the transverse radius of the circumferential dressing wheel apex at the dressing contact point coincides with the dressing axis of the dressing wheel fixture. In a typical dressing wheel fixture this may be accomplished by moving adjustable cross slides in an X-Y plane perpendicular to the dressing axis.
While it may be possible to accurately position certain types of dressing wheels using conventional measuring devices while the dressing wheel is not rotating, there are situations in which the position checking must be accomplished while the dressing wheel is rotating about its center (measurement under dynamic conditions as opposed to static conditions). For instance, in certain diamond dressing wheels the transverse radius of the circumferential apex is not formed by a single diamond but rather by a plurality of diamonds spaced about the circumference of the dressing wheel whose high points define the radial profile while the dressing wheel rotates. Thus, at a given point on the circumference of the dressing wheel a complete radial surface may not be present since the transverse radius is not formed by a single diamond. In such cases static measurements used to attempt to properly locate the dressing wheel result in inaccuracies. Heretofore, it has not been possible to make such measurements dynamically using conventional measuring instruments since such instruments are necessarily destroyed when brought in contact with the rotating dressing wheel. consequently, the dressing wheels must be positioned by a trial and error method which is time consuming and often inaccurate.
Prior to the instant invention, there has long been a need for a positioning system which permits precision location of a dressing wheel under dynamic conditions as well as static conditions.